LCD screens are currently on the market in both commercial and military applications. Such displays are of relatively small display area. There is a need for ever larger displays, with current demands looking at displays on the order of fifteen inches by fifteen inches and larger. Such displays must have great resolution and present a color display. Such requirements present new fabrication challenges that did not exist for the smaller area displays.
LCD's use a liquid crystal to transmit or block the light that creates the image. Liquid crystal is an oily substance that contains thread-like or pneumatic molecules. The liquid crystal compound flows like a liquid, but has a crystalline order in the arrangement of its molecules. These molecules reorient themselves along electric field lines to transmit or block the light that creates the image.
The present invention is useful in fabricating active matrix LCD's (AMLCD's). AMLCD's are essentially a giant integrated circuit with millions of thin-film transistor (TFT) switches fabricated onto a single glass plate. There are several key processes in the fabrication of the TFT layer. The glass plate is purified of alkali metals, which might contaminate the transistors or the liquid crystal. The glass plate is poured so that it is extremely flat. And, the semiconductor layer is deposited by a plasma process which condenses a random network of silicon that is rich in hydrogen on the glass. Finally, metal electrodes, insulators,and other elements in the TFT are deposited in a manner similar to that used in the fabrication of integrated circuits, although the deposition is over a much greater surface area.
The liquid crystal material is sealed between two glass plates, one bearing the TFT's to control the electrodes of each cell, the other bearing color filters and an electrode to complete all the circuits. Each cell is a single color pixel. Each cell must be accurately aligned and there are literally more than a million pixels in a single display. This establishes the requirement for extreme accuracy in alignment of the two etched glass plates.
In final fabrication of the display both the lower and the upper glass plate are etched prior to being placed together to form the display. The two plates are glued together with a spacing medium between the two plates. The spacing medium typically is very small spheres that, prior to gluing the two plates together, are adhered to the underside of the upper plate by static electricity. Strips of glue are placed on the upper side of the lower plate and the two plates are brought close together. The distance between the plates is determined by diameter of the spheres. The glue that is used is sensitive to ultra-violet light. Upon exposure to ultra-violet light, the glue cures, bonding the two plates and producing the finished LCD screen.
It is important to realize that the misalignments of the upper and lower plate can be physically very small and yet produce the very low production yields that are presently being experienced in the industry. Current production of large area AMLCD's is being done mostly in Japan. The best information indicates that alignment of the glass plates is being done by what can be described as the "toothpick" method, that is, the two glass plates are being aligned with respect to one another manually using fine instruments not unlike toothpicks. Estimates of the production yields of the devices are that only ten percent of AMLCD's produced meet specifications. In the past, manufacturing devices for small LCD displays have aligned the upper and lower glass plates in the X and Y planes prior to gluing with adequate accuracy to satisfy the needs of the relatively small LCD's.
The increased size of the plates that are being utilized today requires devices that are capable of aligning such enlarged plates. Additionally, improvement in resolution of such devices requires even greater degrees of accuracy in aligning the two plates in the X, Y and Z planes prior to gluing and while the glue is setting up. In the past, as the glue was setting up, the two plates would frequently shift with respect to one another and thereby degrade the resolution. Additionally, with increased plate size, it is very important that the two plates be uniformly distanced apart in the Z plane as the glue is setting.
Despite the care taken in the production of the two glass plates to ensure that their surfaces are perfectly planar, the two glass plates frequently are slightly wedge shaped. On a fifteen inch square glass plate, the tolerance is .+-.100 microns. In the worst case of two glass plates being positioned together, this would result in a 200 micron spacing between the two at the widest point. This is much too great for usage in a large LCD. For improved resolution, it is desired that the two plates be separated by the exact diameter of the spacing spheres, which are preferably 5 microns in diameter. In practice in the past, if the high point of a wedge on the lower plate aligns with a downwardly extending point of a wedge from the upper plate and a spacing sphere happens to be interposed between those two points, that will determine the minimum closeness of the two plates. Fabrication devices did not allow for pitch or yaw of the two plates with respect to one another to account for the spacing errors caused the wedge shape. This was less of a problem when the plates were substantially smaller in area than those desired to be utilized at the present.
An example of the prior art production devices is described in U.S. Pat. No. 4,923,552. This device utilizes a fixed, descending upper fixture to lower the upper glass plate onto the lower glass plate that is supported by the lower fixture. The two glass plates in their final alignment will only be as parallel with one another as the upper fixture and the lower fixture are parallel. Additionally, an advancing sheet of transparent film is utilized to provide a vacuum seal around the two glass plates while the glue is setting. This is a cumbersome apparatus to utilize and it effectively precludes continuous fine alignment of the two glass plates as the glue is setting.
It would be a decided advantage to have a manufacturing apparatus that would produce relatively large LCD screens with increased production yields.